1. Field of the Invention
The invention relates to an intra-cavity laser beam homogenizer resonator, and particularly to a resonator including one or more bi-prisms for generating a laser beam having a xe2x80x9ctop-hatxe2x80x9d intensity profile along a selected axis.
2. Discussion of the Related Art
A single bi-prism may be positioned in the beam path of an outcoupled laser beam as a homogenizer. The laser beam may have an approximately Gaussian intensity profile as it is incident normal to the flat surface of a bi-prism. As the beam exits the angled surface, the bi-prism splits the wavefront and refracts the two partial beams toward each other. At some distance D past the bi-prism, the intensity profiles at half maximum of the partial beams coincide. At that distance D, the intensity is approximately uniform across the entire beam profile. Beyond distance D, the partial beams diverge from each other and the intensity is no longer uniform. Some related art references include U.S. Pat. Nos. 4,149,773, 4,370,026 (disclosing to use two bi-prisms with orthogonal axes to homogenize in both directions) and U.S. Pat. No. 5,264,412, as well as German published patent application no. DE 39 04 896 A1 and United Kingdom published patent application no. 2 220 502 A (disclosing optionally to use two bi-prisms with orthogonal axes to homogenize in both directions).
German published patent application no. DE 195 33 314 A1 discloses using one bi-prism to homogenize the short axis of an excimer laser beam profile and a second bi-prism to re-collimate the beam. The bi-prisms, oriented with their angled faces toward each other and their axes aligned, are placed in the path of a previously outcoupled excimer laser beam, i.e., extra-cavity. The beam passes through the first bi-prism, where it is split into two partial beams that are refracted toward each other. The second bi-prism is placed where the partial beams have the desired degree of overlap (in this case, the desired intensity profile is somewhat higher at the edges than at the center). The second bi-prism refracts the two partial beams so that they are parallel to the original axis. Both bi-prisms are transmissive optics disposed along the beam path of the outcoupled laser beam such that they are not incorporated into the laser resonator. A shortfall of the DE 195 33 314 A1 published application is illustrated below, wherein a bi-prism homogenizer-collimator advantageously disposed within a laser resonator is described with reference to a preferred embodiment.
Published PCT application no. WO 97/08792 discloses using a pair of bi-prisms and two plane mirrors to direct a laser beam through an amplifier multiple times, with the beam taking a different path on each pass. A mirror is placed behind each bi-prism, and an amplifier is placed at the midpoint between the bi-prisms. An incoming laser beam is incident at an angle on one facet of one of the bi-prisms. The beam is sequentially reflected by the mirrors and refracted by different facets of the bi-prisms, so that the beam passes through the amplifier multiple times before exiting through a hole in one section of one of the bi-prisms. The axes of the bi-prisms are rotated relative to each other so that the beam takes a different path on each pass through the amplifier.
The WO 97/08792 published application describes an arrangement for amplifying an input laser beam including bi-prisms for altering the path of the beam through the amplifying medium on successive passes. The beam is reflected between a pair of mirrors and is refracted by a bi-prism each time it propagates between a mirror and the amplifier. The arrangement described in the ""792 application is not, however, a resonant cavity, and the ""792 application does not disclose an arrangement that can be used to modify the intensity profile of a beam. Instead, a narrow beam is incident on only one facet of the angled surface of a bi-prism on any given pass. Moreover, the multi-pass amplifier disclosed in the ""792 application is configured such that the axes of the bi-prisms are not aligned, so that the light takes a different path on each pass in order to avoid coupling effects. Below, a resonator is described in accord with a preferred embodiment that is advantageous over the resonator described in the WO 97/08792 application, and wherein the axes of a pair of intracavity bi-prisms are aligned.
None of the references mentioned above discloses or suggests using of an intracavity bi-prism or a pair of bi-prisms either as resonator reflectors or otherwise disposed within a laser resonator cavity. It is recognized in the present invention that the preferred embodiments described below including a second bi-prism to collimate the beams after traversing the first bi-prism, and a laser resonator including at least one bi-prism, which is further preferably configured and disposed as a resonator reflector, each provide advantageous laser resonator arrangements for generating laser beams with desired, homogeneous beam profiles not taught or suggested in the related art.
An excimer or molecular fluorine laser system is provided including a discharge chamber filled with a gas mixture at least including a halogen-containing molecular species and a buffer gas, a discharge circuit, multiple electrodes within the discharge chamber and connected to the discharge circuit for energizing the gas mixture, a resonant cavity including the discharge chamber for generating a laser beam, and an intracavity homogenizer for homogenizing an intensity profile of the laser beam generated in the resonator.
The intracavity homogenizer may include a first bi-prism and a second bi-prism disposed at opposite ends of the resonant cavity and having the discharge chamber disposed therebetween, or a first bi-prism and a second bi-prism disposed within the resonant cavity, and either way wherein optical axes of the first bi-prism and the second bi-prism are each at least substantially parallel to the optical axis of the laser beam.
The intracavity homogenizer may include a bi-prism disposed at one end of the resonant cavity, or a bi-prism disposed within the resonant cavity, and either way wherein an optical axis of the bi-prism is at least substantially parallel to the optical axis of the laser beam. The resonant cavity may further include a roof prism, preferably arranged as a resonator reflector.